This invention relates to methods of and compositions for effecting targeted vascular destruction in warm-blooded animals, including humans, and to procedures for identifying drugs capable of such use.
The importance of vasculature to the growth of tumors is an unquestioned scientific reality. Because one blood vessel nourishes thousands of tumor cells, targeting tumor vasculature as a molecular approach to cancer chemotherapies is becoming one of the highest scientific priorities. Two drug models are emerging, i.e., one that prevents the formation of new blood vessels in the tumor (antiangiogenesis) and one that targets vascular destruction as a means of limiting tumor nourishment and/or the impermeability of the lumenal surface of vessel endothelial cells to cancer drugs such as immunotherapies (New England Journal of Medicine 339:473-474, 1998). The antiangiogenic model is basically a cytostatic approach where angiogenic factors generally produced by tumors such as vascular endothelial growth factor (VEGF) and platelet derived endothelial cell growth factor, are blocked by antiangiogenic compounds such as the natural polypeptides angiostatin and endostatin to prevent new blood vessel growth (The Cancer Journal Scientific American 4(4):209-216, 1998; Cell 88:277-285, 1997). On the other hand, the vascular destruction model is a cytotoxic approach where tumor vessels are targeted for cytotoxicity in order to enhance tumor cell cytotoxicity by hypoxia or direct acting chemotherapy.
One of the most potent classes of cancer therapeutic drugs is the antimitotic tubulin polymerization inhibitors (Biochem. Molecular Biology Int. 25(6):1153-1159, 1995; Br. Journal Cancer 71(4):705-711, 1995; Journal Med. Chem. 34(8):2579-2588, 1991; Biochemistry 28(17):6904-6991, 1989). They characteristically have IC50 in vitro cell cytotoxicities in the nM-μM range, but often show poor specificity for killing tumor over normal tissues in vivo, examples of such drugs including combretastatins, taxol (and other taxanes), vinblastine (and other vinca alkaloids), colchicinoids, dolastatins, podophyllotoxins, steganacins, amphethiniles, flavanoids, rhizoxins, curacins A, epothilones A and B, welwistatins, phenstatins, 2-strylquinazolin-4(3H)-ones, stilbenes, 2-aryl-1,8-naphthyridin-4(1H)-ones, 5,6-dihydroindolo (2,1-a)isoquinolines, 2,3-benzo(b)thiophenes, 2,3-substituted benzo(b)furans and 2,3-substituted indoles (Journal of Med. Chem. 41(16):3022-3032, 1998; Journal Med. Chem. 34(8):2579-2588, 1991; Anticancer Drugs 4(1):19-25, 1993; Pharm. Res. 8(6):776-781, 1991; Experimentia 45(2):209-211, 1989; Med. Res. Rev. 16:2067, 1996; Tetrahedron Lett. 34:1035, 1993; Mol. Pharmacol. 49:288, 1996; J. Med. Chem. 41:1688-1695, 1998; J. Med. Chem. 33:1721, 1990; J. Med. Chem. 34:2579, 1991; J. Md. Chem. 40:3049, 1997; J. Med. Chem. 40:3525, 1997; Bioorg. Med. Chem. Lett. 9:1081-1086, 1999; International (PCT) Application No. US 98/04380; U.S. Provisional Patent Application No. 60/154,639). Although tubulin binding agents in general can mediate effects on tumor blood flow, doses that are effective are often also toxic to other normal tissues and not particularly toxic to tumors (Br. J. Cancer 74(Suppl. 27):586-88, 1996).
Many tubulin binding agents such as the combretastatins and taxol analogs are water insoluble and require formulation before evaluation in the clinic. One approach which has been used successfully to overcome this clinical development problem is the formulation of biolabile water soluble prodrugs, such as the phosphate salt derivatives of combretastatin A4 and taxol, that allow metabolic conversion back into the water insoluble form (Anticancer Drug Des. 13(3):183-191, 1998; U.S. Pat. No. 5,561,122; Bioorganic Med. Chem. Lett. 3:1766, 1993; Bioorganic Med. Chem. Lett. 3:1357, 1993). A prodrug is a precursor which will undergo metabolic activation in vivo to the active drug. Stated with further reference to the aforementioned phosphate salt derivatives, the concept here is that non-specific phosphatases such as alkaline phosphatases in mammals are capable of dephosphorylating phosphate prodrugs into the original biologically active forms. This prior art teaches how to administer a water insoluble drug to warm blooded animals for therapeutic purposes under conditions of more maximum absorption and bioavailability by formulation into a water soluble biolabile form (Krogsgaard-Larsen, P. and Bundegaard, H., eds., A textbook of Drug Design and Drug Development, Harvard Academic Publishers, p. 148, 1991).
When the combretastatin A4 phosphate prodrug was used in in vitro and in vivo cell and animal models, it displayed a remarkable specificity for vascular toxicity (Int. J. Radiat. Oncol. Biol. Phys. 42(4):895-903, 1998; Cancer Res. 57(10): 1829-1834, 1997). It was not obvious from this to one skilled in the art that phosphate prodrugs in general, which serve as substrates for alkaline phosphatase, had anything to do whatsoever with vascular targeting. However, the reported data on the combretastatin A4 phosphate prodrug did disclose the principle of preferential vascular toxicity, even though there was no understanding or appreciation of the fact that the prodrug itself was responsible for vascular targeting. In other words, the prior art teaches that A4 and not A4 prodrug was responsible for vascular toxicity by assuming that there was no difference in vascular toxicity between the two forms. The nonobviousness noted above is exemplified by the fact that, although A4 phosphate prodrug and other taxol phosphate prodrugs were promoted as susceptible to phosphatase conversion to the cytotoxic tubulin binding forms, it was never recognized that this enzyme was elevated in microvessels thus targeting them to cytotoxicity.